1. Field of the Invention
The present invention relates to an easily openable liner for a vessel closure and to a closure in combination with the liner. More particularly, the present invention relates to a liner for a vessel closure in which immigration of a lubricant to the surface of a liner formed of a polyolefin type resin is effectively controlled within a certain range at temperatures of a broad range and the opening operation can easily be performed at temperatures of a broad range.
2. Description of the Prior Art
An olefin resin such as low density polyethylene has an appropriate cushioning property and an excellent sanitary characteristic and is widely used as a sealing liner for vessel closures. However, it has been found that when an olefin resin liner is applied to a vessel closure, various troubles arise. For example, when a vessel closure provided with an olefin resin liner is fitted to the mouth of a vessel, such as a glass bottle containing a carbonated drink, and the vessel is stored for a long time, the opening torque of the vessel closure is increased and it is often difficult for a purchaser to open the vessel closure by hand.
As means for reducing the opening torque of the vessel closure, it has been the practice to incorporate a lubricant in the olefin resin to be formed into the liner. This method, however, has not been altogether effective in that the lubricant incorporated into the olefin resin tends to immigrate to the surface of the liner in too great a quantity causing the opening torque of the vessel closure at the initial stage of sealing to be extraordinarily reduced. If the torque of a screwed vessel closure at the initial stage of sealing is excessivly small, insufficient sealing is caused at the initial stage of closing or the vessel closure can become loosened by turning of the vessel closure at the post treatment step or during the transportation, resulting in leakage of the content or intrusion of oxygen from the ambient atmosphere.
Another serious problem encountered with presently available lubricant incorporating olefin resin liners is that the immigration of the lubricant to the surface of the liner is greatly influenced by the ambient temperature. More specifically, in case of an ordinary lubricant, for example, an amide type lubricant for an olefin resin, the higher the temperature, the more accelerated is the immigration of the lubricant to the surface of the liner. At the higher temperatures, the opening torque is thus drastically reduced to cause such troubles as reduction of the seal reliability and sagging of the lubricant from the liner. Also, even though a satisfactory openability is obtained at normal temperatures, the opening torque is increased at low temperatures and opening by hand becomes difficult.